The present invention relates generally to the manufacture of cement and, more particularly, to the use of gypsum and gypsum-based materials used in the manufacture of cements.
Cement generally refers to any material that initially has a plastic form and which is strongly adhesive after hardening. Cement, in the building and engineering context, usually refers to a fine, powdery substance that is processed so that it will adhere together and with other matters and hardens after being mixed with water. Most modem construction cements utilize gypsum, plaster or portland cement. Portland cement is named for a British cement maker, Joseph Aspdin, because of the resemblance that concrete made with his cement to portland stone, a greyish stone that was widely used in England for building construction. Aspdin""s early portland cement was made from lime and clay, or shale, that were heated until they formed cinders and then ground into a fine powder. Those early cements, as with modem portland cements, were mixed with aggregates in the form of sand, or gravel, and water to form concrete, which is the most widely used construction material in the world.
Typically, portland cements are primarily mixtures of tricalcium silicate (3CaO.SiO2), tricalcium aluminate (3CaO.Al2O3), and dicalcium silicate (2CaO.SiO2). These three constituents are used in various proportions along with small amounts of magnesium and iron compounds. When such a cement is mixed with water, hydration occurs in the tricalcium silicate to form a gel-like hydrated silica and calcium hydroxide that eventually crystallizes and binds together with the particles of sand and gravel added to the cement to form a hard mass. This result in hard mass is typically referred to as concrete. Where the mixture contains only a fine aggregate, such as sand, the end product is referred to as portland cement mortar and where the mixture contains both fine and course aggregate, the later being in the form of stone or gravel, the end product is referred to as portland cement concrete.
Each element of the cement and concrete mix affects the overall chemistry of the end product, e.g., how fast it sets, its resistance to chemicals, etc. The tricalcium aluminate used in the cement largely produces an initial set of the mixture but does not contribute overall to the ultimate hardening of the mixture. The dicalcium silicate acts in a similar, deliberate manner. The tricalcium silicate causes rapid hardening of the cement or concrete. Special cements and concretes with special properties may be made by adding or decreasing the proportion of these compounds (and others) to obtain specific properties in the end products.
Gypsum is one such component and an important one at that, because of its effect on the retarding of the hardening process of cements/concretes. Gypsum is hydrated calcium sulfate (CaSO4.2H2O). Gypsum is a cost-effective component that retards the hardening process in portland cement and portland cement concrete significantly to permit the cement or concrete to be delivered, formed and worked while it is in a plastic form. Gypsum acts to reduce the initial rate of heat generation and reduces the rate of hydration of the tricalcium silicate. As an example of its important retarding properties, portland cement that does not use gypsum in its mix will set in about approximately four minutes, while cements/concretes that use gypsum will set in about four hours.
Gypsum is a widely distributed form of a sedimentary rock and it associates with saline deposits such as those formed by the precipitation of calcium sulfate from sea water, as well as with limestone and shale. Gypsum also occurs naturally in volcanic regions and in some clay regions where naturally occurring sulfuric acid has reacted with limestone. Gypsum is mined and crushed at mine locations into aggregate so that it may be eventually transported, usually by rail or truck, to its end user such as a cement plant or a sheet rock manufacturing plant. One by-product of this mining and manufacturing process is the production of gypsum xe2x80x9cfines.xe2x80x9d Fines are very small particles that may pass through 100 mesh sieves. A 100 mesh sieve, as understood in the art and as used in this application is a sieve having 100 openings per inch.
The mass of these fines is comparable to their size and, as such they are very difficult to transport and utilize. Gypsum fines are usually mixed with water to form a slurry and then piped to a disposal area, such as a storage pit. These fines are too small to use in the manufacturing process of portland cement because they are difficult to transport. Additionally, these fines exhibit a natural tendency to clump together and form deposits, or clumps, that agglomerate during their travel through the cement-making process.
In the manufacture of portland cement, raw materials are mixed from deposits of limestone, cement rock, shale, clay, etc., and are crushed into chunks or rocks of the first processing size which are approximately 5 inches in general diameter. These chunks are then crushed a second time down to a suitable storage size which ranges to about 0.75 inches in diameter for separate storage. From there, the storage materials are sent to a grinding mill where they are mixed in appropriate proportions of approximately 60% lime, 19% silica, 8% alumina, 5% iron, 5% magnesia and 3% sulfur trioxide. This mixture is then conveyed to a grinding mill where the matter is ground into a powder and further stored.
This mixed raw material is then heated in a processing kiln in order to form cinders or, as referred to in the cement industry, xe2x80x9cclinker.xe2x80x9d The kilns used for this task may be as long as 500 feet and have a diameter of approximately 12 feet. The kilns are slightly tilted in a horizontal plane. Raw materials are introduced into the upper end of the kiln, either in the form of dry rocks or as a wet paste, and as the kiln rotates, this raw material slowly progresses down to the bottom of the kiln, where an array of burners are located. Hot gases from these burners rise up the kiln to heat and dry the mixture as it progresses down the kiln. As the mixture approaches the base of the kiln, the raw material begins to fuse together to form the aforementioned clinker. In this process, water and carbon dioxide are driven off from the raw material by the kiln temperatures which typically will range from approximately 2700xc2x0 F. to 2900xc2x0 F. Once formed, the clinker is then cooled quickly and ground into a fine powder of about 3000 to about 5000 Blaine, where it may be conveyed by blowers, or the like, to storage silos. This finely ground product is a base portland cement. As mentioned before, the tricalcium silicate in this base cement would rapidly hydrate and harden in anywhere from about 4 to about 10 minutes. Gypsum is typically added at this point of the cement processing to retard the hydration of the cement when it is used in a mix. This gypsum is ground into the clinker during initial clinker grinding in proportions of anywhere between about 3% to about 10% by weight.
The gypsum used by cement plants is usually naturally mined gypsum. In order to control the addition of gypsum to the clinker in the correct proportions, cement plants need gypsum delivered to it in sizes that are easily crushable, for example, in rocks, or chunks, from about 1 to about 3 inches in diameter. These size chunks are easy to transport as compared to gypsum fines. More importantly, chunks or rocks of about this size are easily crushed by the cement processing equipment and because they approximately match the size of the clinker, they are more readily and reliably processed in the final grinding and mixing stage of cement processing.
Synthetic gypsum may also be used in the manufacture of cement. This synthetic gypsum is produced as a waste material during fossil-fired power generation. Stringent air pollution laws mandate limits in the amount of combustion products that are released into the atmosphere by a power generating plant. These limits are important to the power industry which bums fossil fuels, such as oil and coal, in order to form steam and generate electrical power. When a fossil fuel having a high sulfur content is burned, sulfur dioxide (SiO2) is formed as a combustion by-product. In the past, this sulfur dioxide was exhausted to the atmosphere. However, today""s environmental stands strictly reduce the amount of sulfur dioxide emissions that a power plant may release to the atmosphere.
In order to meet these limits, the power industry employs what are known as xe2x80x9cscrubbersxe2x80x9d as part of the exhaust systems on its power plants. These scrubbers utilize a high calcium content lime material which, when contacted with sulfur dioxide-laden air, forms sulfuric acid which reacts with the lime to form a solid material. This process is known as flue gas desulfurization and the resultant by-product is known as flue gas dust (FGD). This FGD is, in effect, a synthetic gypsum having the same chemistry as naturally occurring gypsum, namely hydrated calcium sulfate (CaSO4.2H2O) with nearly the same physical characteristics and properties as naturally occurring gypsum.
These scrubber systems are primarily wet systems, although some may be dry scrubber systems, and thus the FGD formed therein is wet and has the form of a sludge-like material. It is mixed with additional water by power plants so that it may be transported for collection in storage pits or lagoons to dry. Once dried, it is then carted away by the utility as a solid waste to either a landfill, or a sludge pond. Not only does it cost the utility money to have the product carted away, but landfill space is at an ever increasing premium. Therefore, it is very costly for power companies to dispose of their FGD waste in this manner.
Due to the gypsum content of this FGD, it has been utilized in the past in the production of cement. However, this use is not without difficulty in that it is desirable and often critical to have the gypsum, whether it be naturally or FGD-based gypsum, have a consistency so that it may be distributed in the final cement mix in the desire target percentage for the end cement mixture. The FGD has a moist and lumpy nature, when it is taken from a sludge pond, which prevents it from being distributed consistently in cement processing because it not only has a tendency to stick to or bridge in feed systems and hoppers. The wet nature of this FGD sludge causes deleterious sticking and bridging during cement processing and therefore further complicates the use of FGD in cement processing and so cement plants are reluctant to use FGD in such a manner.
It is therefore desirable to provide a means for processing waste gypsum and FGD into a form that may be reliably processed by a cement processing plant. The present invention is therefore directed, in its broadest sense, to a method for processing waste gypsum and FGD by solidifying the waste material under extrusion to both solidify and form the waste material into blocks for use in the processing of portland cement.
Other waste materials that are produced in the manufacture of portland cement can be recycled by the manufacturing plant, such as cement kiln dust and clinker cooler dust, which are particles of cement clinker and which share the same chemical composition as the cement clinker. These two waste particles take the form of a fine powder and present a disposal problem to the cement manufacturer similar to that of a utility and its FGD. Because these two waste materials are of almost the same composition as that of portland cement, they can be recycled into the manufacturing process. The present invention is therefore also directed in another sense to a method for processing waste gypsum and cement plant waste by recycling all of the waste materials into block-like forms that are crushable and hence, usable in the manufacture of cement.
It is therefore a general object of the present invention to provide a means for processing waste gypsum and/or FGD into a solid form that may be used with a high degree of consistency and reliability by a cement manufacturing plant in the processing to a cement mix.
Another general object of the present invention to provide a process for recycling waste gypsum and cement plant waste by extruding into a solid form that may be ground with cement clinker to form portland cement.
Another object of the present invention is to provide a method for solidifying waste gypsum from gypsum fines, FGD or FGD sludge, by extruding it under pressure in order to densify and solidify the gypsum waste into rocks or blocks having predetermined three-dimensional shapes with appropriate predetermined dimensions such that the shapes are easily crushed and ground to a desired consistency and size for use in the manufacture of portland cement.
Still another object of the present invention is to provide novel methods for solidifying waste gypsum sludge, fines, FGD sludge, wallboard waste gypsum, cement waste and fly ash into brick-like forms by mixing the materials together to form a mass, extruding the mass, without adding admixtures to the mass under pressure through a narrow passage to remove free water therefrom and densify the mass in order to obtain a homogeneous, solid composition that is easily transportable and crushable or grindable into predetermined various sizes for use in the processing of portland cements.
Yet a further object of the present invention is to provide a process for reclaiming waste gypsum for eventual use in the manufacture of portland cement, the process including the steps of collecting a predetermined amount of waste gypsum, providing an extrusion die having an opening with a predetermined geometric shape, providing an extruder having an extrusion chamber that defines a torturous path from one end of the extruder to the extrusion die, loading the extruder with the waste gypsum and extruding the waste gypsum through the extrusion die under pressure and vacuum to, in essence, xe2x80x9cdehydratexe2x80x9d or xe2x80x9cdewaterxe2x80x9d the waste gypsum and thereby form a solid, three-dimensional log of xe2x80x9cgreenxe2x80x9d waste gypsum of desired density, the log having matching that of the extrusion die opening, cutting the log of green gypsum in a direction generally transverse to the direction of extrusion to form a block of green waste gypsum, and letting the gypsum block dry to improve its handling characteristics.
A still further object of the present invention is to provide a process for producing solid, crushable objects from waste gypsum that are of a size similar to that of cement clinker for use in the manufacturing of portland cement, the process including the steps of collecting either natural waste gypsum or FGD or a combination of the two, the waste gypsum having a moisture content of about 0% to about 40% by weight, adding water to the waste material to provide a mass of waste material with a desired consistency, dehydrating and densifying the waste gypsum in a single step by passing the waste gypsum through an extruder under pressure and vacuum in order to remove moisture from the waste gypsum so that it has a moisture content of between from about 1% to about 10% by weight (preferably from about 5% to about 8% by weight) after extrusion and densification, passing the so-treated waste gypsum through a die having multiple die openings in order to form an array of xe2x80x9cgreenxe2x80x9d gypsum logs of three-dimensional shape, and cutting the array of logs in directions transverse to the direction of extrusion of the waste gypsum logs to form a quantity of multiple, densified synthetic gypsum waste rocks or chunks similar in size to cement clinker.
Yet another object of the present invention is provide a method for recycling gypsum-based waste materials and cement waste materials for use again by a cement manufacturing plant, the method including collecting gypsum-based waste and mixing it with preselected amounts of cement waste, such as cement kiln dust or clinker cooler dust, the amounts of the cement waste being relatively low in comparison to the gypsum-based waste, so that no substantial hydration and formation of cementitious material occurs, and running the waste mixture through an extruder in order to increase the density of the material by removing a substantial amount of free water from the waste mixture and extruding the densified mass through an extrusion die in order to form a green block of waste material having a preselected three-dimensional configuration, and letting the extruded block dry to eliminate most, if not all, of the free water or water of convenience of the mixture that lies on the surface of the particles and is not adsorbed or used by the particles of the components that make up the mixture.
The present invention accomplishes these objects in a unique and novel manner by providing solid, crushable blocks of synthetic gypsum waste that may be used by other industries, such as in the manufacture of portland cement, and thereby also provides an important process for recycling waste gypsum that otherwise would be deposited in landfills. The gypsum waste is chosen, in the preferred embodiment of the invention, from either natural gypsum waste in the form of gypsum fines resulting from the mining of gypsum, from synthetic gypsum in the form of high calcium sulfate FGD that is collected from flue gas desulfurization equipment. The FGD gypsum waste is collected in a wet, sludge-like form having a moisture content of between about 10% and about 40%.
In another aspect of the present invention, other non-gypsum-based waste products, particularly cement plant waste products, such as cement kiln dust, clinker cooler dust and fly ash may be added in dry form to the waste gypsum. This waste typically has a moisture content of about 0% by weight. The natural gypsum waste has a high percentage of calcium sulfate (CaOSO4), that ranges from about 60% by weight to about 95% by weight, while the synthetic gypsum waste, that obtained from FGD, has a percentage of calcium sulfate that ranges from about 50% by weight to about 95% by weight. The natural gypsum waste (fines) may be mixed with the FGD gypsum waste to raise the final percentage of calcium sulfate to thereby improve the quality of the end product produced by the processes of the invention.
The waste mixture in a sludge form has a moisture content of about 20% to about 25% by weight and is conveyed to an extruder where it is both densified and dehydrated. The extruder preferably is one with one or more center shaft-driven augers and a series of restriction plates. Such an extruder is described in copending U.S. patent application Ser. No. 09/016,587, filed Jan. 30, 1998 and owned by the assignee of the present invention. The extruder dehydrates and densities the waste mixture, removing substantially all of the voids that occur in the waste gypsum and is put under a preselected partial to full vacuum inn order to draw off much of the moisture from the gypsum waste, in order to assist in the solidification and densification of the waste gypsum. The moisture is passed through at least one, and preferably two restriction plates in order to densify the waste gypsum and to even the extruding speed of the waste so that the extruded shapes are not extruded faster at the center of the end product near the auger shaft.
The restrictor plates also assist in removing a desired quantity of some of the moisture present in the waste gypsum so that the final xe2x80x9cgreenxe2x80x9d product that is extruded has a moisture content of between about 1% to about 20% by weight, with the preferred range being between about 5% to about 8% by weight. The extruder dehydrates the waste gypsum by, in effect, squeezing out much of the moisture and air from the waste material under both extrusion pressure and by placing the waste gypsum mixture under a vacuum to remove much of the air voids present in the gypsum waste. It also applies a shearing force to the particles that make up the waste material mass as it passes through the extruder, forcing the particles closer and closer together. This differs from the compression that is known in the art.
The resultant material is a dense, xe2x80x9cgreenxe2x80x9d extrusion in the form of a three-dimensional log, having a density of at least about 1.5 times greater than the density of the waste material entering the extruder. The waste is preferably extruded in an array of green logs, which are subsequently advanced to a cutting station where the array of green logs are cut transversely to the direction of extrusion to create a plurality of green blocks having sizes that approximately match that of cement clinker processed by cement plants. The green blocks are allowed to dry, either naturally or in an oven in order to reduce their moisture content down to an even lower level where the blocks may be easily handled for transport and use without fragmenting or fracturing. Oven drying may be the most appropriate means of drying to use when the process is located on the same site or nearby a cement processing plant and when immediate use of the gypsum blocks is desired.
Other waste materials may be used with the gypsum waste during the extrusion process, such as cement kiln dust, cooler clinker dust and/or flyash. The waste mixture will typically have a moisture content of between about 10% to about 25% by weight, after mixing and after addition of processing water, if needed, which remains as xe2x80x9cfreexe2x80x9d water or water of convenience in the mixture. The waste is conveyed to a mixer where, if desired, wet and dry waste are mixed together in order to at least partially homogenize the waste mixture. It is then conveyed to an extruder that has one or more shaft-driven augers. The augers of the extruders convey the waste material through a restricted and narrow pathway to thereby solidify the waste sludge into a block form.
A vacuum chamber is incorporated into the extruder and preferably positioned between two augers in the extruder order to draw off much of the initial moisture of the mixture, of upwards of about 50% of the total moisture on the entering mixture. The vacuum applied pulls off much of the water as does passing the mixture through a narrow passage and/or restrictor plates. The resultant material is a homogenous, densified synthetic gypsum block or xe2x80x9crockxe2x80x9d, that may be formed into almost any desirable three-dimensional shape, especially shapes with an appropriate aspect ratio that matches that of cement plant grinding mill specifications. Virtually no hydration occurs in the products formed with these other cement waste materials because of their low relative percentages in the overall waste mixture and because the high calcium sulfate content of the gypsum-based waste materials will serve to significantly retard any hydration.
The extruded blocks or logs are somewhat damp when initially extruded. The green blocks may be dried, either in the atmosphere of in a drying oven that is incorporated in the process of the present invention. A suitable time period is relied upon, such as one day, in order to dry, typically by evaporation, of the particle surface moisture on the waste mass. Thus, the extruded blocks will develop a compressive strength in the nature of about 800 to about 1000 pounds per square inch, which strength is perfectly suitable for transport without excessive fragmentation or crumbling, but which is well below conventional structural strengths.
These and other objects, features and advantages of the present invention will be clearly understood through consideration of the following detailed description.